In the series finale of The O.C., a flashforward sequence shows
Seth and Summer getting married.
is walking down the aisle,
she pokes her tongue out at Seth. For a moment he looks surprised or shocked, then moments later stops, as if he remembered some old in-joke between them, rendering the conventional meaning of the tongue-poke to be moot.
What does this any of this signify? I didn't understand either the supposed meaning of the tongue-poke or Seth's eventual realisation of its true meaning in context.